The present invention relates to hydrodynamic bearings and disk recording/reproducing apparatuses equipped with them.
Disk recording/reproducing apparatuses include magnetic disks and magnetically or optically perform reading and writing of data for the magnetic disks while revolving the magnetic disks. Further increases in capacity and speedups of data transfers are required of disk recording/reproducing apparatuses. Accordingly, it is desired that revolutions of the magnetic disks become still faster and are stabilized with still higher precision. Hydrodynamic bearings are suitable for such high-speed and high-precision rotary drive systems.
FIG. 6 is a cross-sectional view showing an example of conventional hydrodynamic bearings. The top end of a shaft 31 is fixed on the center of a hub 36. A flange 33 in an annular shape allows the bottom end of the shaft 31 to pass through its inside and is fixed at the bottom end of the shaft 31. Thrust dynamic-pressure generating grooves 33A and 33B are provided on surfaces of the flange 33. An outer surface of a sleeve 32 is fixed on a base 35, and an inner surface 32A of the sleeve 32 surrounds the shaft 31. The flange 33 is then placed in a hollow 32D formed by a bottom surface of the sleeve 32 and an inner surface of the base 35. A thrust plate 34 is fixed on the base 35 and closes the lower side of a space surrounded by the sleeve 32 and the base 35. The upper surface of the thrust plate 34 is then opposed to the lower surface of the flange 33. In this hydrodynamic bearing, in particular, the thrust plate 34 completely cuts off gaps among the flange 33, the sleeve 32, and the base 35 from the outside space. Radial dynamic-pressure generating grooves are provided on one or both of a side of the shaft 31 and an inner surface of the sleeve 32. Radial dynamic-pressure generating grooves are usually provided on two regions, a first region 32B near the flange 33 and a second region 32C near the upper opening end of the sleeve 32 (see broken lines shown in FIG. 6.) The thrust dynamic-pressure generating grooves 33A and 33B and the radial dynamic-pressure generating grooves 32B and 32C are, for example, herringbone-shaped grooves. Gaps among the shaft 31, the sleeve 32, the thrust plate 34, and the base 35 are filled with oil 42. Magnetic disks 39 are fixed on the outer surface of the hub 36, being concentric with the shaft 31. Generally, several sheets of the magnetic disks 39 are installed. The spacers 40 are installed between inner radii of the magnetic disks 39, and the clamper 41 further presses down the inner radii of the magnetic disks 39 from the top. Thereby, the magnetic disks 39 are fixed on the hub 36. Magnets 38 are installed on the inner surfaces of the hub 36. On the other hand, stators 37 are installed on the base 35 and opposed to the magnets 38.
The above-described hydrodynamic bearing operates as follows. Rotating magnetic fields occur when the stators 37 are energized. The hub 36 undergoes a torque from the rotating magnetic fields through the magnets 38. Thereby, the shaft 31, the hub 36, and the magnetic disks 39 revolve in a body around the shaft 31. During the revolution, the oil 42 flows along the radial dynamic-pressure generating grooves and is concentrated in each central part of the first region 32B and the second region 32C. As a result, pressure in the radial direction of the shaft 31 is enhanced in those central parts. This pumping effect maintains stable spacing between the shaft 31 and the sleeve 32, and thereby the rotation axis of the magnetic disks 39 does not substantially shift in the radial direction of the shaft 31. Similarly, the oil 42 flows along the thrust dynamic-pressure generating grooves 33A and 33B and is concentrated in each central part of regions where the thrust dynamic-pressure generating grooves 33A and 33B are provided. As a result, pressure in the axial direction of the shaft 31 is enhanced on surfaces of the flange 23. This pumping effect maintains stable spacing between the flange 33 and the sleeve 32 and stable spacing between the flange 33 and the thrust plate 34. Therefore, the rotation axis of the magnetic disks does not substantially tilt from the axial direction of the shaft 31. Thus, the above-described hydrodynamic bearing maintains the high-speed revolution of the magnetic disks 39 stable with high precision.
In such a conventional hydrodynamic bearing as the above-described one, the above-described pumping effects are fully exerted under the condition with the oil 42 covering the whole of the radial dynamic-pressure generating grooves 32B and 32C and the whole of the thrust dynamic-pressure generating grooves 33A and 33B. However, an abundance of minute air bubbles (microbubbles) intrudes into the oil 42, for example, after a time lapse of use. The microbubbles accumulate particularly in spaces where pressure is low among gaps filled with the oil 42, and then agglomerate into large air bubbles there. FIG. 7 is a cross-sectional view showing positions where the air bubbles tend to appear. The air bubbles 43 tend to accumulate in the intermediate region 32E between the first region 32B and the second region 32C, the perimeter of the flange 33, and their vicinities, as shown in FIG. 7. When those air bubbles are large and many, or when those swell with variations of outside air pressure or temperature rises of the oil 42, the oil 42 is pushed and shifts by the pressure of the air bubbles. Thereby, the oil 42 tends to escape outward from the gap between the top of the shaft 31 and the upper opening of the sleeve 32 (see droplets 42A shown in FIG. 7.) Furthermore, a so-called lack of oil film, that is, a condition that the oil 42 fails to cover the whole of the radial dynamic-pressure generating grooves and the thrust dynamic-pressure generating grooves, occurs when the amount of leakage of the oil 42 is excessive. In that case, the above-described pumping effects become insufficient, and this increases, for example, the risk of excessively hard contact between the shaft 31 and the sleeve 32 or between the flange 33 and the thrust plate 34 resulting in serious wear of them.